This invention relates to a rotation transmission device mounted on a drive train of a motor vehicle for changeover between transmission and shutoff of the driving force.
When a four-wheel drive vehicle turns on a paved road with the front and rear wheels directly coupled together, the vehicle will experience a negative phenomenon known as tight corner braking. In order to prevent such a problem, the applicant of this invention proposed rotation transmission devices including a roller type two-way clutch and an electromagnetic coil in unexamined Japanese patent publication 10-71872.
FIGS. 1 and 2 show a rotation transmission device A of this type, which is a part of a transfer case 5 of an FR-based 4WD vehicle having hub clutches 2 at the outer ends of front wheels 1. The transfer case 5 includes an input shaft 6 through which the output of a transmission 4 coupled to an engine 3 is delivered directly to a propeller shaft 8 for rear vehicle wheels 7, and a chain sprocket 9 coaxially and relatively rotatably mounted on the input shaft 6. The rotation transmission device A includes a roller-engage type two-way clutch 10 for selectively transmitting the rotation force from the input shaft 6 to the chain sprocket 9, and an electromagnetic clutch 11 for selectively locking and unlocking the two-way clutch 10. The rotation transmission device adds a 4WD control mode (AUTO-MODE) to the drive modes available in the typical part-time 4WD vehicle (which are 2WD, 4WD-Hi and 4WD-Lo).
FIGS. 3A and 3B show in detail the two-way clutch 10 and the electromagnetic clutch 11 of the rotation transmission device A. The two-way clutch 10 includes an inner member 12 and an outer ring 13 coaxially and relatively rotationally mounted around the inner member 12 through bearings. One of the inner and outer members 12, 13 is formed with a plurality of cam faces 14, while the other is formed with a cylindrical surface 15 opposite the cam faces 14 to define a wedge-like space therebetween. Mounted in the wedge space is a retainer 16 having a plurality of pockets each accommodating a roller 17 as an engaging element. The rollers 17 are biased toward a neutral position, i.e. a position in which they do not engage the cylindrical surface 15 or the cam faces 14, by a switch spring 18 having ends thereof engaged by the retainer 16 and one of the inner member 12 and the outer ring 13 that is formed with the cam faces 14.
The electromagnetic clutch 11 comprises a friction flange 19 fixed to the outer ring 13 or the inner member 12, an armature 20 mounted on an end of the retainer 16 so as to be slidable but not rotatable relative to the retainer 16 and to be in juxtaposition with the friction flange 19 with a suitable clearance present therebetween, and an electromagnetic coil 21 for magnetically pressing the friction flange 19 and the armature 20 against each other. Thus, by activating and deactivating the electromagnetic coil 21, the rollers 17 engage and disengage.
In such a system, it is desired that when AUTO-MODE is selected with a mode changeover switch 22, the two-way clutch 10 is locked as quickly as possible when an ECU 23 (electronic control unit) begins to apply a voltage to the electromagnetic coil 21.
FIG. 7 shows the time lag between the activation of the electromagntic clutch 11 and the locking of the two-way clutch 10 in a conventional system. The electromagnetic coil 21 begins to produce an electromagnetic force slightly after point of time t0, i.e. the time when the ECU 23 detects slippage of one of the rear wheels and begins to apply a voltage to the electromagnetic coil 21. As the electromagnetic force increases with time, the frictional torque produced between the armature 20 and the friction flange 19 increases, and when the frictional torque exceeds the torque of the switch spring 18 in the two-way clutch 10, the clutch is locked.
In the conventional control method, it takes a long time before the two-way clutch locks after the ECU 23 detects slippage. Due to this delay in response, if the vehicle is started sharply quickly on a low-friction .mu. road such as on a frozen or otherwise slippery road, the rotation speed difference between front wheels 1 and rear wheels 7 will widen by the time the two-way clutch 10 locks. As a result, when the two-way clutch 10 locks belatedly, a large shock is inflicted on the vehicle.
An object of this invention is to provide a rotation transmission device which can prevent a large shock when the vehicle is started quickly on a low-.mu. road.
To shorten the response time from the detection of a slip by the ECU 23 to the lockup of the two-way clutch 10, a large current has to be supplied to the electromagnetic coil. The use of a high-power electromagnetic coil is, thus, unavoidable.
In a conventional control method, when the travel mode has been changed over to the LOCK mode, a voltage (say 12 V) is constantly applied to the electromagnetic coil. Thus, during the LOCK mode, the power consumption and heat buildup are large.
Another object of this invention is to provide a system for controlling a rotation transmission device which consumes less power during the LOCK mode, and which is turned off while the parking brake is applied, thereby reducing the power consumption as a whole.